Begging intensity per nestling is thought to increase with brood size as a result of intra-brood competition for the resources brought by the parents. This in turn should increase the extent of call overlap within the brood. The overlap of calls from different individuals will create an acoustical interference resulting in a masking effect of the acoustic signal. Begging call overlap may also increase the overall volume of the begging calls, potentially magnifying the effect of the begging message to the receptor (parent). On the other hand, this may increase the risk of the nest being detected by a predator. In this study we describe the effect of brood size on nestling vocal begging behaviour in the Spanish Sparrow Passer hispaniolensis. We tested the hypothesis that chicks in larger broods would show a lower per capita begging rate, duration and vigour. Indeed, both the vigour of the initial response to the arrival of the parents and the vocal begging intensity per nestling were lower in larger broods. This would result in lower call overlap in larger broods, but since nestlings in larger broods called longer, the extent of call overlap per nestling did not vary with brood size. Although not final, these results seem to support a possible effect of acoustic interference in nestling vocal begging behaviour. Nevertheless, this is only one possible explanation and some of the findings could also be the result of cooperative synchronization of sibling calls to increase parental care.